3. 3
CHAPTER 1
1.1. Introduction
Many current applications in fluvial geomorphology are based on the importance of the
effective flow and bankfull flow. According to Wolman and Miller (1960, p.55) which states
that “effective” discharge refers to the range of flow magnitudes that transports the majority
of a river’s annual sediment load over the long-term whereas further research done by
Leopold et, al. (1964, p.522) suggest, that “bankfull” discharge is the flow magnitude that is
contained within a channel without overtopping its bank. These two factors are seen as the
driving force in channel form and biotic function (Rosgen, 1996). The importance of the
bankfull flood is demonstrated by its essential position in many classification systems and
natural-channel restoration designs and applications (Montgomery and Buffington, 1998).
Channel morphology is a major variable in fluvial systems (Rowntree and Wadeson, 1999).
Channel change can occur for two reasons. It can occur both naturally (over short and long
time periods) and as a result of anthropogenic modification to rivers or their catchments (eg.
impoundments, water transfers, agriculture).
Floods however can change the morphological components of a river immediately. Floods
are natural events but extreme floods are known to accomplish large morphologic changes in
a channel and move large volumes of sediment (Fitzpatrick and Knox, 2000). According to
Rowntree and Wadeson (1999) “floods are the direct response to heavy or prolonged storm
events which are very important in geomorphological processes as medium to high
discharges are needed for significant fluvial system morphological changes”. A large flood
that exceeds a channel forming threshold has the ability to induce a large amount of
morphologic change (Schumm, 1977). For example, a large flow exceeding the channel
forming threshold forms features, such as meandering cut offs. This commonly occurs during
an extreme flood where flow height exceeds the bank height at the cut off and rapid erosion
ensues as the channel develops a shorter, steeper new course. This changed course is
commonly unstable until the channel adjusts to the new slope by altering erosion rates or
other factors to reach a stable form (Schumm, 1977). In systems where stream banks do not
have a typical mature alluvial floodplain, the processes may not follow these normal patterns.
For example, in a river system where the channel is undercutting a high terrace, large floods
may produce catastrophic erosion and major channel change as large volumes of sediment are
injected into the channel. Earlier research done in the sixties by (Schumm and Lichty, 1965)
4. 4
found that once the terrace is undercut and exposed as a steep unstable bank even smaller
subsequent flows may continue to inject anomalous high volumes of sediment. In order to
understand how floods operate in river system, understanding the link between morphology
and processes helps decide if processes exist to create the morphology. Recent research done
by (Church, 1992) found that understanding the link between morphology and the sediment
system, helps to diagnose problems from channel form. Further research by Church (1992)
found that the implications of creating different morphology and understanding the link
between morphology and physical habitat/ecology which provides the link between
biodiversity goals and geomorphology. According to Thorn et, al. (1996) when these
operations in a river system are understood other components of river morphology changes
may be established.
1.2. Problem statement
The abnormal high discharge rates between August - November 2013 caused a major flood
event in the Lourens River. Flood events usually cause major channel morphology changes.
In the case of the Lourens River changes can be observed by comparing the channel changes
of September 2014 with before channel changes.
1.3. Researchquestion
The questions asked in the investigation would be:
What can be regarded as a major flood?
Can the discharge during the period under investigation be considered / regarded as a
flood event?
Can the observed changes be quantified?
Are there any indications of damage in morphology of the channel and if changes
occurred where in the channel did it occur?
1.4. Objectives
To define what exactly we mean by a “flood” (Literature survey).
The second objective would be to evaluate the discharge of the Lourens River (during
the study period) to see if it can be called a “major flood” event by analysing the
discharge and rainfall data.
5. 5
The third objective would be to quantify the changes that have occurred by making
comparison of cross – sections before and after flood transections.
The fourth objective would be to investigate the Lourens River channel and identify
areas where changes might have occurred.
1.5. Aim of the study
The aim of this study is to determine the effect the major flood event of 2013 had on the river
morphology of the Lourens River, Somerset West.
1.6. Hypothesis
It is hypothesised that abnormal high discharge will lead to visible changes in channel
morphology.
1.7. Significance of the study
The significance of the study is that river systems change on numerous times throughout the
year. The relevance of the study is to make a useful contribution to understanding the
mechanism of changes in a river channel and to help save the riparian zone which is used as a
recreational site. By taking these relevance in consideration the relationship between
morphology and flood hydraulics can be best understood, which increases the knowledge of
the impact floods can have on channel morphology of the Lourens River.
1.8. Outline of the report
The outline of the thesis is to investigate the river morphology changes of the Lourens River,
Somerset – West after a major flood event. These changes should be evident after the flood
event of 2013. By assessing the changes, preventive and conservative measures can be made
in the future in order to reduce damaging of the environment and loss of life.
6. 6
CHAPTER 2
LITERATURE REVIEW
2.1. Literature
Criteria for defining a “flood” vary in definitions, but in general all imply overbank flow
(Wolman and Miller, 1960). Throughout time, floods have altered the floodplain landscape.
These areas are continuously shaped by the forces of water - either eroded or built up through
deposit of sediment (Peppler, 2006). When a river is flooded there are expected geomorphic
responses (Peppler, 2006). Although all floods are represented by a rise of river stage, the
magnitude and characteristics of geomorphic impacts vary significantly with flood
characteristics. Rare, high-magnitude floods are capable of producing spectacular channel
changes and movement of coarse sediments (Baker, 1977; Gupta, 1988).
Floods are the direct response to heavy or prolonged storm events (Rowntree and Wadeson,
1999). They are very important in geomorphological processes as medium to high discharges
are needed for significant fluvial system morphological changes (Rowntree and Wadeson,
1999; Rowntree, 2000). Extreme floods are known to accomplish large morphologic changes
in a channel and move large volumes of sediment (Fitzpatrick and Knox, 2000). Baker (1977)
and Kochel (1988) states, that both catchment controls and channel controls are important
variables influencing the role of floods of differing magnitude and frequency. According to
Wolman and Miller (1960) both flood magnitude and flood duration are important for
understanding the impacts of floods on erosion, sedimentation and river morphology. Flood
runoff is the most important in determining geomorphological processes as high discharges
are required for significant sediment entrainment and transport (Rowntree and Wadeson,
1999). A short duration flood with a high discharge will produce more damage than a small,
less intense flood. It is widely accepted that the increasing bed load transport and intensive
dune and bar migration during floods have an effect on cross-sectional area (Németh, 1954).
The size of a river channel is governed by the water flow through it, particularly flood peak
flows that affect erosion and deposition. Many people have associated bankfull channel
dimensions with floods. Many have also associated bankfull discharge with the most
effective flows for sediment transport. Bartholdy and Billi (2002) investigated on influences
of flood severity on river morphology in part of the Sesina River in Italy and concluded that;
small flood event transfers river meanders downstream. Bartholdy and Billi (2002) further
found that on large rivers that it combine some meanders and remove some of them, which
7. 7
cause the decrease of sinuosity, number of meanders and also development of straight
patterns in river system. However, Church (1992, p.130) makes the point that “there is no
universally consistent correlation between bankfull flow and a particular recurrence interval
or between flood frequency and effectiveness in creating morphological change”. This arises
because rivers with different calibre bed material require different discharges for sediment
transport and bank erosion. Further research done by Simon (1995) suggested that a scour
and fill process develops eventually when bank erosion appears after a heavy flood event
over periods of hours to days.
Beaumont (1981) reported that the removal of catchment and channel vegetation increased
large floods which resulted in significant channel erosion and enlargement, with the
previously meandering channel shifting to a straighter channel. According to Friedman et, al.
(1996a) floods often have a persistent effect on bottomland morphology and vegetation in
some regions and in small catchments. A later study by Friedman et, al. (1996b) found that
floods with differing sizes and durations are likely to impact riparian systems in different
hydrologic and geochemical ways. Recent research by Meehl, et al. (2007) found that a well-
informed understanding of how floods impact riparian hydrologic processes and water quality
will become increasingly important if predictions of increased precipitation intensity over
much of the globe are realized. According to Meehl, et al. (2007) a process-based
understanding of flood groundwater interactions will be especially valuable because, as flood
frequency, intensity and duration change in individual riparian systems, processes not
initially identifiable or important to a particular riparian system (but perhaps observed
elsewhere) may emerge as critical local hydrologic and/or biogeochemical drivers.
According to Allan and Soden (2008) global climate projections indicate that future increases
in precipitation intensity are likely to be more severe which in return is expected for flooding
to be more severe. Soden (2000) defined precipitation “as the general term for rainfall,
snowfall, and other forms of frozen or liquid water falling from clouds”. Precipitation is
intermittent, and the character of the precipitation, when it occurs, depends greatly on
temperature and the weather situation. Consequently, floods could play an increasingly
important role in driving riparian hydrologic processes throughout much of the world as
suggested by (Meehl, et al., 2007). While floods are extreme and uncommon events, their
hydrologic and geochemical consequences can influence riparian systems long after flooding
ends. Floods are often fairly local and develop on short timescales (Soden, 2000). Research
done by Allan and Soden (2008) at a later stage found that local, sudden floods (flash floods)
8. 8
describe flooding in small catchments is mainly caused by short and highly intensive
precipitation. Flash floods occur primarily in hilly or mountainous areas due to prevailing
convective rainfall mechanisms, thin soils and high runoff velocities (Bronster, 1996). The
time for these events is short. In general, the duration of the flood event is also short, but this
flood type is frequently connected with severe damages.
In fluvial areas the morphology is expressed in terms of plan form, longitudinal profile and
cross section and changes in these characteristics are driven by variations in discharge and
sediment transport in rivers and additionally by tidal currents, density-driven currents, wind
induced currents and waves. According to Thorne (1997) the morphological response to these
drivers is dependent on the boundary conditions of discharge, sediment load, valley slope and
topography, channel roughness; the bed material, bank material and in-channel and bank
vegetation. Rivers are constantly adjusting and evolving in response to the sequences of
normal flow, flood flow and drought events which are associated with regional climate, local
weather and catchment hydrology (Thorne, 1997).
There are a range of geomorphological classification systems which make qualitative links
between channel process, form and stability. Thorne (1997) gives an overview of alternative
theories on classifying channel morphology. The approaches for understanding channel
morphology aim to relate the cross section, slope and/or planform to characteristics of
stability, sediment type and valley landform (Thorne, 1997). The literature also provides
some debate about the evolution of morphology and styles of channel change. In addition to
morphological change induced by natural processes, the activities of people on the floodplain
and their use and management of rivers and the water environment cause morphological
adjustment as found in Thorne (1997) on his research on “Channel Types and Morphological
Classification”. Human activities within the catchment and land use change may influence
the nature of the runoff regime and the sediment budget. The construction of embankments
and flood defences on the floodplain influence the functioning of the natural river processes.
Direct interventions in the channel such as the construction of structures, plan form
modifications and bed and bank stabilisation measures influence the fluvial
geomorphological processes while the capital works are being carried out and following the
works. Channel maintenance activities such a dredging and vegetation cutting also influence
the natural processes and will result in modifications to the natural channel morphology.
9. 9
Morphological change in rivers is influenced by flood hydraulics and so too, is flood
hydraulics influenced by morphological change. Although there has been much research into
the influence of flooding on morphological evolution, studies to understand how
morphological change can influence flood hydraulics and, in turn, flood risk are not
widespread. It is arguably this aspect of the relationship that has most impact on the way in
which society must learn to live with rivers as flood risk directly influences risk to life,
property, infrastructure and the environment. With an increased knowledge of the impact that
morphology can have on flood risk, it is possible to implement more appropriate management
strategies to deal with morphological change that has an impact on flooding and, therefore,
potentially mitigate the likelihood and the consequences of flooding. This rationale has
motivated the research reported here which is to investigate the morphological changes of the
river in to response to abnormal discharge and to quantify whether the abnormality can be
considered a flood event. This will be proven through parameters of rainfall, discharge data
cross sectional measurements and photographic monitoring.
10. 10
CHAPTER 3
STUDY AREA & METHODS
3.1. Introduction
This chapter contains a detailed description of the study area and methodologies. This section
is divided into two parts. The first part of the section is to provide a description of the study
area and the selected reach. The second part is to, observe hydrological variation through
historical rainfall data and discharge data coupled with the methods of cross-sectional
measurements and photographic monitoring.
3.2. Study area
The Lourens River rises in the Hottentots Holland Mountains at an altitude of about 1 080 m
(Figure 1). The catchment of the Lourens River is surrounded by the Helderberg and
Hottentots Holland mountains (Figure 1).The Lourens River runs through Somerset and
flows in a south-westerly direction for 20 km before discharging into the ocean at False Bay.
The length of the river is approximately 20 km. Tharme, et al. (1997) found that the
catchment is approximately 140 km3. The Lourens River lies within the winter rainfall region
(Heydorn & Tinley, 1980). The mean annual rainfall is approximately 1200 mm for the
region, whilst the mean annual evaporation is calculated at 1410 mm (DWAF, 2003). The
study area received very high precipitation during the winter months of 2013. However
November of that year shows an abnormal high yield of rainfall (Figure 3). According to
research done by Tharme, et al. (1997) which stated that the estimates of naturalised mean
annual runoff (MAR) for the catchment were in the order of 122 x 106 m3 of which 87%
occurs in winter while only 13% occurs in summer.
Figure 1: Location of the Lourens River catchment area within the Heldergberg.
11. 11
Figure 2: Location of the study reach close to Radloff Park, Somerset West (Google Earth, 30 August 2013).
Figure 3: Rainfall data for the study area (Lourensford farm, 2013)
The Table Mountain Group sandstones underlie the upper slopes and the Pre – Cape,
Malmesbury Group shales and greywacke underlie the middle slopes of the Lourens
catchment (Cliff and Grindley, 1982). The Lourens River catchment area falls within the
Fynbos Biome (Davies and Day, 1998). Water canals running from the high catchment areas
on the farm of Lourensford are densely inhabited with natural vegetation such as bullrushes
and fragmitis (Bryant, 2008). The canals lead into attenuation ponds where further settling
and filtration takes place. The natural vegetation in the downstream foothill and coastal plain
12. 12
has been replaced by mixed forestry, agricultural crops, pasturelands and a mixture of
residential, industrial, urban and recreational developments. The river contains indigenous
fish such as Sandeliacapensis and Galaxia spp while alien trout and carp were released in the
past (Bryant, 2008). Shy Cape clawless otters can be seen at night, while marsh mongoose,
giant mongoose and Cape grey mongoose live off the fish (Bryant, 2008). The riparian zone
is covered by invasive herbaceous wood plant species such as grey popular (Populus x
canescens), wandering jew (Commelinabenghalensis), etc. These alien vegetated plant
species cause the reduction of indigenous species particularly within the riparian zones of the
Lourens River (Tharme et al., 1997).
The study reach is located in the foothill zone and close to a recreational area, Radloff Park at
a location of (-34o04’58.84’’S, 18o52’12.50’’E) (Figure 1, 2). The study area consists of
features which include stony substratum, small sized cobbles to relatively medium to large
cobbles/boulders throughout the reach. The upper section of study reach consists of fast
flowing waters over a predominance of smaller cobbles with finer cobbles washed on a point
bar (Plate 1a, b). Irregularities in the channel are caused due to the big point bar in the river
which causes eddies with fast flowing waters (Plate 1b). The river flow changes from a
run/riffle hydraulic biotype to a fast flowing eddy at the upper section to a deep a pool/riffle
at the middle and lower section (Plate 2a, b). The Reach morphology could be classified as
fast riffle/eddy/pool/riffle run hydraulic biotopes.
Plate 1: Photographic comparison between the (a) upper section (predominance of smaller cobbles with finer
cobbles) and (b) middle section (point bar and fast flowing eddying with backwaters) of the study reach on 24
July 2014.
a b
Left bank
Right
bank
Left bank
Point bar
13. 13
Plate 2: (a) Photographic view of river flow changes from a run riffle and eddying hydraulic biotype to (b) a
pool/riffle run biotype at the lower section.
The study reach was chosen due to itsaccessibility, diverse array of geomorphologicaland
physical characteristicsand degree of disturbance (King et al.,2003). The study reach lacked
major channel modifications such as canalisation or impoundments. The upper section (see
plate 2a) a large tree is falling over which is primarily due to the instability of the root system
that is closely located to the rivers flow regime. The flow path of the river is causing a
undercutting of the lower left bank as you look across from the right bank. The right banks in
the lower section as you look downstream are steep and covered with kikuyu grass growing
on the lower banks of the river (see plate 2b). The right bank is mainly cut away on the edge
of the river as the flow changes from an eddying hydraulic biotype to a pool/run biotype and
flows past the right bank at the edge of the base of the bank.
3.3. Methods
Possible channel morphological changes were assessed during 2014 by using historical
rainfall data over a 97 year monthly average and comparing the data to a monthly total of
2013. A similar method was applied to the discharge data. The Discharge data of 3 respective
weirs on the river were compared to each other. Discharge data for weirs 2 (G2HO29;
G2HO16) averages (2000 – 2009; 1985 - 1990) were compared and the last weir (G2HO44)
Point bar
Left bank
Right bank
Falling tree
Left bank
14. 14
monthly average (2005 - 2012) was compared to the monthly total of 2013. The second
parameter, a cross sectional measurement made by obtaining data through surveying and
comparing pre - flooding data of 2013 with channel morphological measurements made in
September 2014 and lastly photographic monitoring of the study reach twice a month (upper,
and lower section).
3.3.1. Historical rainfall data
Rainfall data was collected from a neighbouring farm (Lourensford) located in the upper
reaches of the river. Monthly rainfall data was collected by the farm owner over a 97 year
period. A monthly 97 year rainfall average would be compared to the 2013 monthly rainfall
data. From these graphs differences in rainfall (mm) can be distinguished and the significance
of the rainfall events of 2013 can be quantified in comparison to the monthly 97 year average
rainfall for the area.
3.3.2. Discharge data
Secondary data collected by the Department of Water Affairs was accessed via online
database on the 18 of August 2013. The discharge data was collectedfrom 3 weirs along the
river (see Figure 1). The data was collected for weirs (G2H044, G2H016 and G2HO29).
Each weirs data was presented in a line graph. Weir G2HO44 discharge average (2005 -
2012) was compared to monthly total of 2013, whereas weir GH2O16 a monthly discharge
average (1985-1990) was presented and the same procedure was done for weir G2HO29 a
monthly average (2000 - 2009) was presented.
3.3.3. Cross-sectional data
Seven cross sections were surveyed during pre-flooding conditions in 2013 at the study site
during low flow conditions. The seven cross sections were surveyed again in September 2014
using the same predefined points of 2013 as a reference mark. The channel transactions were
surveyed with a theodolite, a Leica 100 series model. Setting up a theodolite is carried out in
three stages: Centring the theodolites; levelling the theodolites and the removal of parallax.
The theodolite is to be centred over a nail in the top of a peg. This is a typical point or
reference mark. The tripod is first set up over the peg. The legs of the tripod are placed an
equal distance from the peg and are extended to suit the height of the observer. The tripod
head should be made as level as possible by eye. Standing back a few paces from the tripod,
the centre of the tripod head is checked to see if it is vertically above the peg – this should be
done by eye from two directions at right angles. If the tripod is not centred, each leg is moved
15. 15
a distance equal to the amount the tripod is judged to be off centre and in the same direction
in which it is not centred. It is important to keep the tripod head level when changing its
position. When the tripod has been centred in this way, the tripod legs are pushed firmly into
the ground. If one foot goes in more than the others making the tripod head go off level, this
can be allowed for by loosening the clamp of the tripod leg affected, adjusting the length and
then re-clamping. The theodolite is taken out of its case, its exact position being noted to help
in replacement, and it is securely attached to the tripod head. The ground mark under the
theodolite is now observed through the optical plummet. The plummet is adjusted such that
the nail in the peg and the plummet’s reference mark are seen together in a clear focus
(Gordon et al., 1992). By adjusting the three foot screws, the image of the nail seen through
the plummet is moved until it coincides with the reference mark. The circular bubble on the
tribrach is now centred by adjusting the length of individual tripod legs. To level the
instrument exactly, the plate level is used. The theodolite is rotated until the plate level axis is
parallel to the line through any two foot screws. Lastly the parallax is eliminated by
accurately focusing the cross hairs of the telescope against a light background and focusing
the instrument on a distant target. Through the telescope, three readings were taken, which
were the top value, middle value and the bottom value. Readings from the theodolite were
taken from a start point, from one side of the bank (right bank) across the channel to the
opposite bank (left bank). The distance to the channel, which is the top value minus the
bottom value, should always be increasing. If these conditions were not met, it may indicate
that the readings or measurements were wrong and not accurate. The readings were taken in a
straight line. The procedure was carried out for all seven cross – sectional measurements
made at the study reach.
3.3.4. Photographic monitoring
Field observations were made twice a month by taking photographic images of the study site
at beginning of a month and at the end. These photographic images were taken from June –
September 2014. The photos were taken from the same fixed points and using a Garmin –
etrex (GPS) to locate the specific locations. The reason for taken these images was to observe
channel changes from early winter - summer. Photos were taken along the river where
changes could be expected such as hydraulic biotopes of run, riffle and pool. Photographic
images were also taken of bank vegetation on bank left and right bank that best capture any
possible channel changes.
16. 16
CHAPTER 4
RESULTS
4.1. Introduction
The aim of this chapter is to present the results from the analysis of different data and method
sets. This section is divided into four parts. Part one deals with the analysis of comparing a
monthly 97 year average (1917 - 2013) rainfall to the monthly total of 2013. Part two deals
with the analysis of discharge rates of 3 respective weirs. Part three deals with analysis of
pre- and post-flooding cross-section measurements. Part four deals with photographic
monitoring of the upper and lower section twice a month between June – September 2014 to
quantify if any possible channel changes had occurred.
4.2. Historical rainfall data
The monthly rainfall total of 2013 was compared with the monthly 97 year average rainfall
data. It is evident based on (Figure 4) that there’s an abnormal amount of rainfall (mm) in
2013 compared to the monthly 97 year average rainfall. 2013 shows high rainfall (mm) for
August and November 2013 whereas the average rainfall of the monthly 97 year average
shows a low indication (seasonal pattern) of rainfall for the Lourens River. The trend
observed on Figure 4 indicate that the norm (97 year average) distribution of high rainfall for
the catchment is between June – August each year, however in 2013 the amount of rainfall
for certain months show an irregular pattern distributed across the year. The months of
February, April, June, August, September and November shows higher values in amount of
rainfall (mm) then the average (1917 - 2013) rainfall for the catchment. The rainfall for 2013
shows a trend that does not follow the seasonal rains as observed from the average amount
for the catchment. Months of June, August and September shows figures of double and triple
the amount of the monthly 97 year average the catchment receives (see table 1).
17. 17
Rainfall (mm) comparison between a 97 year average and 2013
Difference in precipitation (mm)
97 years (avg) 2 013 between (97 year (avg) - 2013)
January 27 24 -3
February 25 95 70
March 32 29 -3
April 77 128 51
May 129 98 -31
June 161 203 42
July 149 137 -12
August 140 362 222
September 92 134 42
October 62 28 -34
November 46 220 174
December 28 9 -19
Figure 4: Graph representation comparing the precipitation (mm) of the monthly 97 year average (1917 - 2013)
to monthly total of 2013 (Lourensford Farm, 2013).
Table 1: Comparing the monthly rainfall (1917 - 2013) to the monthly total of 2013.
The amount of rainfall indicated by table 1, suggest that the amount of rainfall were very
variable across the year of 2013. The difference in rainfall (mm) shows an irregular trend
with rainfall difference being of minus values in certain months and positive values in other
months. February of 2013 showed a 70 mm difference from the monthly 97 year average
rainfall. Rainfall irregularities can be further seen by decreasing and increasing values
represented by April and May of 2013. The Monthly 97 year average rainfall shows a steady
incline from April (77 mm) to May (129 mm), however the monthly total of 2013 shows a
decreasing values between the two months (128 mm – 98 mm). The monthly 97 year
averages according to the table 1 clearly show a steady rainfall pattern with values increasing
gradually from January - April and a steady incline from May - August which indicates the
18. 18
MONTHS JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
YEARS
2005 0.335 0.121 0.029 1.96 2.03 9.73 5.22 9.38 4.3 2.51 1.24 0.284
2006 0.17 0.064 0.052 0.512 3.28 3.46 5.42 6.4 2.51 2.52 2.72 0.836
2007 0.28 0.233 0.802 1.33 3.33 8.29 8.85 12.4 5.92 4.91 4.63 1.84
2008 0.905 0.312 M M M 0.562 11 5.13 17.1 6.82 5.71 2.13
2009 1.25 0.418 0.404 0.399 3.15 9.23 8.24 7.41 7.78 5.46 9.04 3.19
2010 0.929 0.689 0.812 0.379 6.38 5.52 4.46 4.7 2.98 3.56 2.04 0.71
2011 0.191 0.054 0.031 0.627 2 4.01 2.69 3.66 4.5 1.78 1.74 0.77
2012 0.209 0.067 0.261 0.825 1.57 3.78 7.03 12.2 8.17 8.13 2.63 0.925
Average discharge rates (2005-2012) 0.53 0.24 0.34 0.86 3.11 5.57 6.61 7.66 6.66 4.46 3.72 1.34
Monthly total discharge rates of 2013 0.415 0.49 0.567 2.71 2.72 6.55 6.43 15.2 14.1 5.28 8.59 2.67
winter season for the Lourens River area. However the monthly total of 2013 shows
abnormality in rainfall as there is no clear pattern that compares to the 97 year average
rainfall of the area. As mentioned above rainfall was variable for all the months 2013 except
January and March which seemed very similar In the amount of rainfall (mm) as 97 year
average for the catchment. In August (362 mm) of 2013 the monthly total showed a
difference of 222 mm in comparison to the monthly 97 year average rainfall, clearly
displaying an abnormality in the amount of rainfall (mm) the area received that month. In
November the monthly rainfall (mm) difference was 174 mm in comparison to the 46 mm
average the area received over the 97 years. The Lourens River is characterized by a seasonal
rainfall regime of about three months (wet season) from June - August with peak rainfall
amount in the month of June and lowest rainfall in the month of January, May and December.
June contributes on average (1917-2012) about 161 mm of rainfall annually.
4.3. Discharge data
4.3.1. Weir (G2HO44)
Discharge data of weir (G2H044) monthly average (2005 - 2012) was compared to the
monthly total of 2013. The monthly average for each year is calculated in the table, however
between the months of March – May 2008 data was indicated as missing “M” (see table 2).
The monthly average (2005 - 2012) discharge rates in table 2 show a steady trend in readings
across the 7 years averages. Discharge readings in the months of August, September and
October 2013 are much higher than the average discharge rates over the 7 years. Discharge
rates for these 3 months (August = 15.2 m-3s-1; September = 14.1 m-3s-1; October = 8.59 m-3s-
1) were abnormal compare to the average discharge rates of the river over 7 years.
Table 2: Discharge rates of weir (G2HO44) comparing monthly average (2005 - 2012) to the monthly total of
2013 (DWAF, online database).
Table 2, is represented by the graph (see figure 5) which shows the average (2005 - 2012)
discharge rates and the monthly total of 2013. As mention above the average (2005 - 2012)
19. 19
discharge rates depicts a steady trend. The trend is evident on the graph as values from
January – April (0.53m-3s-1; 0.24m-3s-1; 0.34m-3s-1; 0.86m-3s-1) which indicate base
flow/normal flow for the river as values show similarities. From June – October (5.57m-3s-1;
6.61m-3s-1; 7.66m-3s-1; 6.66m-3s-1; 4.46m-3s-1) show a similar pattern for 7 years (2005 -
2012). This show clear indication that the catchment experiences high discharge rates in
winter and low discharge rates in summer. However the monthly total of 2013 values show
irregularities in discharge values across the year. Discharges are similar at the beginning of
the year to the average discharge (2005 - 2012), but values further in the year of 2013 are
double the discharge rates for the catchment. Discharge rates in 2013, August (15.2 m-3s-1)
which is evident of peak discharge is a 7.54 m-3s-1 more than the average (2005 - 2012)
discharge rates as well as the month of September (14.1 m-3s-1) which show a discharge rate
difference of 7.44 m-3s-1 which is far greater than the 6.66 m-3s-1 average discharge calculated
between 2005 - 2012. The discharge rate of November 2013 was estimated at 8.59 m-3s-1. The
discharge rate is 4.87 m-3s-1greater than the expected 3.72 m-3s-1 for the catchment, which
shows that the month of November experienced discharge values which were abnormal for
that particular time of the year.
Figure 5: Graph representation comparing discharge rates (m-3s-1) of weir (G2HO44) monthly average (2005 -
2012) to the monthly total of 2013 (DWAF, online database).
4.3.2. Weir (G2HO29 and G2HO16)
The monthly discharge average (2000 - 2009) of weir (G2HO29) and the monthly discharge
average (1985 - 1990) of weir (G2HO16) were compared to each other. The data presented in
table 3 & 4, suggest that discharge values between the two weirs are fairly similar in each
20. 20
MONTHS JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
YEARS
2000 0.355 0.29 0.636 0.203 1.67 2.03 0.361 1.24 9.83 2.71 0.683 0.17
2001 0.28 0.127 0.236 0.496 2.36 3.16 17.1 16.6 16.3 6.34 2.49 0.469
2002 2.86 0.613 0.342 1.28 4.68 8.43 11.8 10.4 5.84 4.26 1.59 0.538
2003 0.268 0.284 2.07 0.42 1.4 0.555 1.61 10.6 8.77 6.77 0.903 1.3
2004 0.806 0.202 0.68 2.29 0.465 3 3.35 13.6 4.8 5.5 2.08 1.18
2005 1.63 0.818 0.595 4.78 5 13.9 9.16 14.7 9.15 5.59 3.54 1.56
2006 0.905 0.564 0.616 1.76 7.46 6.25 10.4 13.4 6.89 6.32 5.71 3.11
2007 1.66 1.85 3.15 4.54 8.68 14.8 15 18.9 12.3 9.4 10.1 6.18
2008 3.75 2.22 2.75 2.83 6.49 5.02 16.1 9.63 17.7 11.7 10.2 5.69
2009 3.25 1.46 2.19 2.82 7.27 13.5 15.2 16.8 15.5 12.1 14.1
Monthly discharge average (2000-2009) rates 1.58 0.84 1.33 2.14 4.55 7.06 10.01 12.59 10.71 7.07 5.14 2.24
MONTHS JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
YEARS
1985 0.636 0.733 1.93 2.21 2.64 5.51 8.05 7.53 4.28 2.41 0.813 0.286
1986 0.073 0.116 0.498 2.13 3.39 5.51 8.36 17.2 9.76 3.84 2.26 1.43
1987 1.57 1.33 1.41 0.945 6.54 7.24 7.36 14.1 12.9 4.3 1.53 1.85
1988 0.553 0.341 0.555 2.24 3.31 4.18 7 4.71 7.3 3.36 1.55 0.908
1989 0.639 0.67 2.66 3.02 5.16 6.9 9.24 11.2 12.2 7.37 3.12 1.2
1990 0.835 2.93 0.456 3.32 5.43 6.41 15.2 11.4 4.29 1.23 0.81 0.475
Monthly discharge average (1985-1990)rates 0.72 1.02 1.25 2.31 4.41 5.96 9.20 11.02 8.46 3.75 1.68 1.02
month of the different averages respectively. Values of both weirs in every month seem to
follow the same pattern even though the monthly discharge averages are calculated at
different time scales of (1985 - 1990) and (2000 - 2009) respectively.
Table 3 & 4: Table representation of the monthly discharge average (2000 - 2009) of weir (G2HO29) &
monthly discharge average (1985 - 2000) of weir (G2HO16) (DWAF, online database).
The graphical representation of both weir (G2HO29) and (G2HO16) show that the two weirs
show similarities in discharge rates with both weirs discharge starting from a relative low
discharge (see Figure 6a, b). Discharge rates at the beginning of the year display a steady
range from January - April for both weirs averages which indicate base flow for the river. A
steady incline can be observed from May – August. The highest discharge rates at both weirs
are evident in the month of August which is evident of the peak discharge of the catchment
regardless of the different time scale. Discharge slowly decreases from the months between
September – December for both weirs, which indicate that the river discharge is returning to
base flow/normal flow.
Figure 6(a): Monthly discharge average (2000-2009) rates of weir (G2HO29) (DWAF, online database).
21. 21
Figure 6(b): Monthly discharge average (1985-1990) rates of weir (G2HO16) (DWAF, online database).
4.4. Cross – section change
The cross – sections of the study site indicate that the channel was slightly sinuous and stable
in 2013 based on (see Appendix A, Figure. 8a-g) cross sectional measurements. The Cross
sectional bank on the right were all assumed to be stable at the starting points, which had
made it relatively easily in doing the measurements. The river has an active channel which
incise into a much bigger macro-channel which was more pronounced on the right bank as
you look downstream than on the left bank. The general trend of the cross sections can be
observed through its width with the upper section (cross section 1.1 - 1.5) showing high
variability in width, ranging from 3m to 12m (see Appendix A, Fig. 8a-e). The lower section
(1.8 - 1.9) (see Appendix A, Fig. 8f-g) display a less variable but narrower range, with values
ranging between 1m and 4m. It can be seen from Fig.8, cross sections 1.1 to 1.5 that major
channel morphology changes occurred within the upper section of the study reach.
The changes observed at the upper section are evident due to the high/abnormal rainfall (mm)
of 2013 as well as the high discharge rates experienced from August – November (see Figure
4, 5). These changes were not caused by a single high flow or rainfall event rather a few
events throughout the year (King et al., 2003). Cross section 1.3 to 1.5, the channel expanded
due to severe erosion and the retreat of the active left bank, (see Appendix A, Fig. 8a-e).
Cross section 1.3 on the left bank demonstrates active undercutting of the bank due to the
river flow regime which led to instability of the left bank (Plate 3a). The high rainfall (mm)
coupled with the wide range of discharge rates changed this particular part of the upper
section on a large scale. It is evident that the changes at 1.4 and particularly 1.3 were due to
slumping of the left bank that occurred at times of high rainfall which caused peak discharges
22. 22
downstream. Deposition and sediment in the channel can be seen at all the cross sections (see
Appendix A, Fig. 8a-g). Channel deepening is demonstrated at all cross sections (see
Appendix A, Fig. 8a-g), but with cross section 1.3 which show the active channel retreating
more to the left bank in comparison to 2013 pre - flooding cross section. At cross section 1.4
from the right bank to the left bank deposition took place with most significant deposition
appearing on the point bar. An average of about 0.65m – 1m deposited on the point bar at
cross-section 1.4 (see Appendix A, Fig. 8d; Plate 3b).
It is evident that the upper section of the study site underwent major channel morphology
changes with both processes of deposition and erosion present. These processes contribute to
both deepening of the channel and a build-up of areas adjacent to and in the main channel. In
contradiction, the downstream cross-sections (1.8 and 1.9) channel geometry was less
variable and much more stable with little change in channel form. This was evident to
observe as only sediment was eroded away on the right bank (see Appendix A, Fig. 8f-g).
This is mainly due to the high elevation of the bank and the result of dry bank sediment that
produces more cohesive banks, which is capable of withstanding heavy winter or prolonged
rainfall. The section also shows that the active channel of the river changed from being
narrow to one with noticeable banks on either side with channel deepening occurring across
the channel. The banks of both cross-section 1.8 and 1.9 (see Appendix Fig. 8f-g) indicate no
clear scouring on the bed or banks, which indicate that the sediment were supplied from
upstream reaches.
Plate 3:(a) Demonstrate active undercutting and left bank retreat on the outer bend (lateral migration) at cross
section 1.3 on the left bank and (b) deposition on the point bar at cross-section 1.4 (c) meandering bend is
present and the process of undercutting evident (cross section 1.5) due to the point bar that cause eddies with
fast swirling waters.
Cross section
1.5 and
meandering
bend
Point bar
cb
Point bar
bar
23. 23
4.5. Photographic monitoring
Field observations were made at 3 different fix points (see table 5). Photos were taken at the
beginning and the end of each month from June – September 2014. Photographs were taken
between cross section 1.4, 1.5 and 1.8 respectively (see Appendix A, plate 5,6a-h and 7a-f).
Cross section 1.4 and 1.5 showed the major changes in channel morphology as observed at
Plate 3a, c (see section, 4.3.Cross-section change). Plate 4(a-h) shows the monitoring of the
point bar at the beginning and end of the month. From plate (4a) it can be seen that fast
swirling waters and quiet backwaters are evident. This can be further be seen at plate (5a)
which shows the fast flowing waters and quiet backwaters from an higher elevation. The
evidence presented indicates that from 5 July – 29 September (see Appendix A, plate 4c-h)
the rivers flow velocity changed over time and the river was starting to run empty as base
flow was present in the month of September due to boulders that are visible in the channel.
This is well supported by plate 5d-h, which shows from an aerial view how the rivers flow
velocity changed as boulders and cobbles could be observed at plate 5h. Plate 4-5h and 6f
indicate dense vegetation which implies that over time, not only did the flow change but the
vegetation on both left and right bank changed. In contrast the downstream changes were less
variable, with the hydraulic biotype changing from a pool/riffle run biotype (see Appendix A,
plate 6a, 6b) to a riffle hydraulic biotype as depicted by plate (6f).
.
Table 5: Representation of the latitude, longitude and elevation of 3 different surveyed sites where photographic
monitoring took place.
Latitude Longitude Eelvation
Photographic monitoring
Plate 5 (a-h) S 34.0885 E 18.87099 ELV-57m
Plate 6 (a-h) S 34.08290 E 18.87078 ELV-56m
Plate 7 (a-f) S 34.08299 E 18.87055 ELV-55m
24. 24
CHAPTER 5
DISCUSSION
5.1. Historical rainfall data & Discharge data
The direct relationship between rainfall and discharge of the Lourens River surmises the fact
that the river discharge is a response to the amount of rainfall as depicted in (Figure 7). From
January – May 2013, a moderate rainfall and discharge are observed which indicate base flow
for the river. The rainfall and discharge from June – August starts increasing, with August
marking the peak rainfall (362 mm) and discharge (15.2 m-3s-1). The lag time is the difference
between time of the heaviest rainfall and maximum discharge of the river. The short lag time
as seen from Figure 7, could be due to the characteristics of the river basin. The relief of the
Somerset West catchment has relatively steep slopes. Meaning that, the faster water flows
overland into the river, the quicker peak discharge is reached in return with high rainfall
given a short lag time as observed in August. From September however a declining trend is
observed in rainfall but not in the discharge. In August heavy rainfall occurred, which may
have waterlogged the ground enabling infiltration and with water instead traveling via
overland flow or through flow. This antecedent rainfall condition (rainfall that has already
happen) saturates the ground. Pore spaces are filled and can no longer infiltrate these heavy
rainfalls, causing saturated overland flow into the river which cause the high discharge of
14.1 m-3s-1 for September. This is well backed up by the literature, where research done by
Allan and Soden (2008) stated that, flooding (flash floods) in small catchments is mainly
caused by short and highly intensive precipitation. After such an event runoff will occur
quickly where the surface is impermeable, eg impermeable rocks/concrete causing a sudden
increase in discharges as observed in the month of September.
Another reason for the high discharge in September could be the basin size. A large river
basin will collect more water than a smaller one, potentially leading to a higher peak flow.
As there are larger areas to cover, the lag time will be greater in a larger basin. However,
Tharme, et al. (1997) found that the catchment is approximately 140 km3 in size and this
indicating that the catchment which is relatively small exceeded its monthly 97 year average
rainfall (Figure 4). The variability of shear stress, which is determined by difference of flow
and channel slope, could have contributed to the high discharge for September. The high
discharge of September could have been caused by the high shear stress (same discharge as
August due to same channel geometry) of the river.
25. 25
The Lourens River lies within the winter rainfall region which characterized by a seasonal
rainfall regime of about three months (wet season) this is evident in Table 1 and Figure 4 –
“monthly 97 year average rainfall”. However the seasonal regime observed in “figure 7” for
the basin does not agree with the rainfall regime for the catchment as earlier stated by
Heydorn & Tinley (1980). A decline in rainfall and discharge is observed in October,
however the month of November shows an abnormally high rainfall and discharge, which is
uncharacteristic for this particular time of the year as wet season for the region is between
June – August. The difference between maximum rainfall and peak charge is practically the
same for August indicating a greater risk of flooding. The peak rainfall and discharge of
November coupled with the antecedent rainfall of August and characteristics of the drainage
basin could have caused major changes in channel morphology in the Lourens River. A full
detailed study for these processes and mechanism appears in the research done Allan and
Soden (2008) on the “Atmosphere warming and the implication of precipitation extremes”.
Figure 7: Graph representation comparing total monthly rainfall of 2013 to the total monthly discharge of 2013
(G2HO44) (Lourenford farm 2013; DWAF, online data base).
The abnormality of the discharge rates is evident when comparing the discharge rates of weir
(G2HO44) to weir G2HO29 and G2HO16 respectively. Although both of these weirs
G2HO29 (2000 - 2009) and G2HO16 (1985 – 1990) are in different time scales they both
indicate the same pattern of discharge for the catchment (see Figure 6a, b). Both weirs show
that the highest discharge rates are between June – August which indicate the wet season for
the catchment as rainfall is expected to be high too. However the discharge rates of weir
(G2HO44) in 2013 shows no true comparison to the other two weirs when the rainfall season
is, as discharge rates are abnormally high in certain months. This gives a true reflection that
26. 26
there is a strong relationship between rainfall and discharge of the Lourens River catchment
in 2013 and that the maximum rainfall and discharge could cause a flood event.
5.2. Cross-section change
The channel morphology assessment was performed on the 28 September 2014. In the study
it is proposed that the degree and rate of the observed channel adjustments and bank retreat
were most likely caused by heavy rainfall and high discharge rates from the months of
August – November 2013. As observed in Figure 7 the amount of rainfall and discharge for
August (362mm; 15.2m-3s-1) and November (220mm; 8.59m-3s-1) were an abnormal high for
the Lourens River catchment. The heavy rainfall paired with high discharge in turn as well
affected the erosion and deposition processes. Other factors that could have played a role in
channel morphology changes at the study reach are the characteristics of the Lourens River
catchment. At the upper section of the river past the study reach on the left bank vineyards
dominate the land which could have resulted in a shift in runoff regime. Another reason could
be the enormous scale urbanization has taken place at the upper sections of the catchment
with houses being build close to the river floodplain. Thorne (1997) as well indicate that,
channel maintenance activities such a dredging and vegetation cutting also influence the
natural processes and will result in modifications to the natural channel morphology.
A period of heavy prolonged rainfall between August – November 2013 at the study site
resulted increase discharge rates downstream in the Lourens River channel (see Figure 5).
This is evident, as discharge rates were an abnormal high between August – November 2013
for weir (G2HO44). In November the discharge rates was a 7.54 m-3s-1 more than the average
(2005-2012) which was a minimal 3.72 m-3s-1 over 7 years. The same could be said about the
months of August and September respectively. As indicated in table 2, discharge rates were
substantially higher than the averages. August had a monthly total of 15.2 m-3s-1and
September had a monthly total of 14.1 m-3s-1. The discharge rates difference presented were
equal to the averages over the 7 years for the catchment. These differences of 7.54 m-3s-1 for
August and 6.66 m-3s-1 September 2013 is evident of peak discharge rates which was caused
by heavy rainfall for the area (see figure 4, 5). The two events were more likely attributable
to channel morphology changes of the river. These discharges exerted higher shear stress
(same discharge and channel geometry) on the river bed and banks and resulted in channel
instability at the upper section cross section 1.3, 1.4 and 1.5 (see Appendix A, Fig. 8c-e).
27. 27
Evidence was presented in the form of channel and bank migration, channel widening and
narrowing, scour and fill of bank and bed, channel deposition and increase deposition on
point bar formation. Severe left bank erosion led to the active channel being increased from 3
m to 5m m in width (see Appendix A, Fig. 8a-d). The active channel not only increase in
width but changed in depth. A comparison between pre – and post flooding cross sections
1.1-1.9 show an incision of the channel bed which led to an increase depth of approximately
0.25 – 0.5m (see Appendix A, Fig. 8a-g). The processes of wet bank slumping and bank
undercutting resulted in bank erosion (Plate 3a). This is evident from the analysis of cross-
section 1.3 (see Appendix A, Fig. 8c & Plate 3a) which shows at a vertical distance of 63m
on the left bank a piece of the left bank was eroded away (approximately about 2m)
indicating the erosion of the bank. Bank retreat results when the flow scours the bed at the
base of the channel bank to bring about gravitational failure of the lower bank that’s in tacked
(Thorne, 1990). A detailed account of these processes and mechanisms appear in (Thorne
1982; 1990) and Knighton (1984). These process most likely operated interdependently from
one another, although undercutting was perhaps more significant.
Drying (contraction and expansion) and wetting (swelling) of the left and the lack of
stabilising of non – cohesive root systems caused the outer blocks from left bank to be pushed
away from the bank to such an extent cause slumping of blocks onto the base of the bank and
most certainly into the channel when the root system can no longer hold the weight of the
massive block (Plate 3a). The incision is evident at cross-section 1.3 (see Appendix A,
Fig.8c) where there is a massive incision of a depth of about 1.25 m at a horizontal distance
of 63 m before the block. Wet bank slumping took place at cross section 1.5 (see Appendix
A, Fig. 8e) as a meandering bend is present and the process of undercutting evident due to the
point bar that cause eddies with fast swirling waters (Plate 3c) . From cross-section 1.1 – 1.9
(Fig7a-g) undercutting of the lower bank is evident at both upper and lower sections, which
meant that undercutting contributed to bank instability, resulting in further bank erosion and
sediment input into the river. Coupled with high flow velocity of a river, undercutting
coincides with floods on a river; therefore heavy rainfall did not only cause the severe bank
losses. It was rains and discharge in association followed by the drawdown of the river that
caused really marked changes (Rowntree, 2000). A comparison between pre- and post-flood
cross-sections 1.3 and 1.4 (see Appendix A, Fig. 8c and d) which clearly indicate a lateral
shift in the plan form to the left. Bank erosion resulted in a left bank retreat (increase in
channel with). The erosion of left bank and channel widening played a role in the observed
28. 28
change in channel plan form. Continued left bank migration was evident during field visit
from the beginning of the study period until the end of the study period (see Appendix A,
Plate 4, 5a-h).
5.3. Photographic monitoring
In the previous two section of 5.1 and 5.2 it was clearly discussed what the influence heavy
rainfall and high discharge are on river morphology changes. Photos were taken from specific
points on the river where channel changes could have been expected between June –
September 2014 by monitoring these three respective sites through photographs (Table 5). It
was widely expected that the high rainfall and discharge rates between August – November
2013 caused a flood event, which in return changed the morphology of the river. The photos
taken from June – September 2014 could validate the assumption made above. As seen from
Plate 4a-f (see Appendix A), which is a photographic monitoring trend of the point bar, which
is located between cross – section 1.4 and 1.5. Flow velocity changes are evident based on
plate 4a-h. Flow changes from being fast at the beginning of June to decreasing slowly from
the 5 July until the 29 September. The flow regime is responsible for eroding, transporting
and depositing channel sediments (Scheepers, 2014). From the photographic monitoring of
plate 4a-f it is evident that the flow caused the depositing of sediment on the point bar. This is
evident by observing Fig8d, cross – section 1.4 which indicates the deposition of sediment
between 0.65m – 1m. The plate trend from 4a-h shows clearly how deposition took place
from June – September 2014. The second monitoring site took place at cross section 1.5 (see
Appendix A, Plate 5a-h) straight across the channel looking from the right bank to the left
bank. As previously mentioned the flow regime is an observed changing factor in all three
monitoring sites. The observations made from June – September 2014 at cross section 1.5
(Plate 5a-h) indicate a few characteristic changes. The firs one is the hydrological biotype of
the river changes completely over the few months. The reach morphology was classified in
chapter 3 (Study area, Plate 2a, b) as fast riffle/eddy/pool/riffle run hydraulic biotype,
however based on plate 5a-h (see Appendix A) it is evident changes had occur.
Hydraulic biotypes of this section of the river changed to a run/eddying/riffle/pool run
biotype (see Appendix A, Plate 6a-e). Another observed change at cross section 1.5 is the
deposition of cobbles and boulders on the point bars edge (see Appendix A, Fig5, a-f). As the
magnitude of the flow regime changes of the river the deposition of small finer cobbles are
evident on the point bar edge. When the river has a high flow of water, cobbles and boulders
are orientated throughout the section, however when the magnitude of water flow is low,
29. 29
these smaller finer cobbles deposits on places which is adjacent to the active channel of the
river. Another contributing factor which causes the deposition of sediments on the point bar
is the meandering bend at cross section 1.5. The meandering of the river causes eddies with
fast flowing waters and backwash. The irregularity of the meandering bend coupled with flow
regime of high magnitude could have caused the deposition of sediment (cobbles, boulders
etc.) on the point bar (seen Appendix A, Plate 5a-h).
Based on Plate 6a-f, which is located at cross section 1.8 looking upstream, not much
observed changes, were evident from the observational monitoring. As mentioned above,
hydraulic biotype of the upper section changed but the lower section of the river stayed the
same biotype of pool/riffle run with only the flow regime changing during the surveying
period. A noticeable change from plate 6a-f is the change of vegetation which becomes
denser by each monitoring photo (see Appendix A, Plate 6a-f). It could be surmised that
photographic monitoring/surveying started at the beginning of the winter month (June) and
ended in an early summer/spring month (September) for the study site. These changes are
quantified based on photographic monitoring and successive field visit during the study
period and not analysis of rainfall and discharge data as the data for the year 2014 were not
available at the time of the study period.
30. 30
CHAPTER 6
6.1. Conclusion
It was hypothesised that abnormal high discharge will lead to visible changes in channel
morphology at the study site. The high rainfall and discharge between the months of August –
November 2013 had a significant impact on the channel morphology changes of the Lourens
River. Based on the findings of this study and in light of the objectives outline in chapter one,
the following conclusion can be made. Channel morphology changes at the study site were
both influenced by meteorological causes and hydrological changes. The result of this study
strengthens the understanding of a strong relationship between rainfall and discharge as high
rainfall caused high discharge rates which contributed to a flood event in 2013. Bed and bank
erosion, channel migration and narrowing, point bar deposition and formation were the result
of abnormal high rainfall and discharge changes which effectively caused the flood event,
which changed the channel morphology. Other contributing factors could as well contribute
to the observed changes. The catchment characteristics (slope, surface type, basin size etc.),
urbanization around the floodplain, deforestation, and land use changes could have
contributed to changes in channel morphology. Changes were more visible within the upper
section of the study area, with the lower section being more stable, showing less changes
from the right bank to the left bank.
6.2. Limitations of the study
The study was conducted within the constraint time period of five months. The following
limitations were experienced:
1. The research was only conducted during the wet season therefore a comparative study
was impossible between wet and dry season.
2. Water uptake by various vegetation species were not measured or monitored.
3. Cross sectional measurements were done in late September, due to safety concerns as
river flow was strong in the earlier months of the study period.
4. Some of the pegs of the cross sectional points of the previous study were missing
therefore it was time consuming to find them even though GPS coordinates were
available for them.
31. 31
References
Allan, .P., and Soden, B.J., 2008. Atmosphere warming and the implication of precipitation
extremes, science, 32pp, 1481-1484, doi 10.1126/science. 1160787.
Baker, V.R., 1977. Stream-channel response to floods, with examples from central Texas.
Geol. Soc. Am. Bull. 88pp. 1057-1071.
Bartholdy, J. and Billi, P., 2002. Morphodynamics of a pseudo meandering gravel bar reach,
Geomorphology, 42:pp. 293–310.
Beaumont, R.D., 1981. The effect of land-use changes on the stability of the Hout Bay River.
Mun. Eng. 12 (2), 79-87.
Bronster, A., 1996. River flooding in Germany: influenced by climate change? Physics and
chemistry of the earth, 20(5-6), pp. 455-450.
Bryant, D., 2008. Flood havoc predicts 15 year ago. DistrikPos, [online] Available at:
http;//www. HartklopandieHeldeberg.com/popsci/html> [Accessed 18 July 2014].
Church, M.A., 1992. Channel morphology & typology, in Calow, P &Petts, GE (Eds).The
river handbook; hydrological and ecological principles, Oxford: Blackwell Scientific
Publications.
Cliff, S., and Grindley, J.R., 1982. Report No 17: Lourens (CSW7). In: Heydorn AEF and
Grindley JR (eds.) Estuaries of the Cape, Part II. Synopsis of Available Information on
Individual Systems. CSIR, Stellenbosch, Cape Town.
Davies, B.R., and Day, J., 1998. “Vanishing Waters”. UCT Press, Cape Town, South Africa.
487 pp.
DWAF, 2003. State-of-Rivers Report, River Health Programme. Diep, Hout Bay, Lourens
and Palmiet River Systems.
DWAF, (nd). Hydrology (Data, Dams, flood, flows). [unverified data] Hydrological services.
G2HO (Doc numb). Strand – Somerset West. Site: Department of Water Affairs [online
database].
Rhoda, F., (2015). The channel morphology and Ecology changes of the Lourens River. M.s.c
thesis, unpublished.
32. 32
Fitzpatrick, F.A., and J.C. Knox, 2000. Spatial and temporal sensitivity of hydrogeomorphic
response and recovery to deforestation, agriculture, and floods, Physical Geography, v. 21 (2)
pp. 89-108.
Friedman, J. M., W. R. Osterkamp, and W. M. Lewis, Jr. 1996a. Channel narrowing and
vegetation development following a Great Plains flood, Ecology, 77:2167–2181.
Friedman, J. M., W. R. Osterkamp, and W. M. Lewis, Jr. 1996b. The role of vegetation and
bed-level fluctuations.
Google Earth 6.0. 2013. AfriGIS (Pty) Ltd, Lourens River 34o04’58.84” S, 18o52’12.50” E,
elevation 62m. Available through: <http//www.google.com/earth/index.htm/< [Accessed 30
August 2014].
Gordon, N.D., McMahan, T.A., and Finlayson, B.L., 1992. Stream Hydrology: An
Introduction for Ecologists. John Wiley and Sons, Chichester. 526 pp.
Gupta, A., 1988. Large floods as geomorphic events in humid tropics, in Flood
Geomorphology, edited by V.R. Baker, R.C. Kochel, and P.C. Patton, pp. 301-315, Wiley,
N.Y.
Heydorn, A.E.F., and Tinley, K.L., 1980. Estuaries of the Cape.Part I. Synopsis of the Cape
Coast. CSIR Research Report 380. 97pp
King, J.M, Scheepers, A.C.T, Fisher R.C., Reinecke, K.M. and Smith L.B., 2003. River
Rehabilitation: Literature Review, Case Studies and Emerging Principles. WRC Report No
1161/1/03. Water Research Commission, Pretoria, South Africa.
Knighton, D., 1998.Fluvial Forms and Processes.A New Perspective.Arnold Publishers,
London. 383 pp.
Kochel, R.C., 1988. Geomorphic impact of large floods: review and new perspectives on
magnitude and frequency. In: Baker VR (ed.). Flood Geomorphology. New York ,Wiley.. pp.
169-187.
Leopold, L.B., Wolman, M.G., and Miller, P., 1964. Fluvial Processes in Geomorphology.
edition. W.H. Freeman and Company, 522 p.
33. 33
Meehl, G.A., Stocker, T.F., Collins, W.D., Friedlingstein, P., Gaye, A.T., Gregory, J.M.,
Kitoh, A., Knutti, R., Murphy, J.M., Noda, A., Raper, S.C.B., Watterson, I.G., Weaver, A.J.,
Zhao, Z-C., 2007. Global Climate Projections. in: Climate Change 2007: The Physical
Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change.(ds., Cambridge University Press, Cambridge,
United Kingdom and New York, NY, USA.
Montgomery, D. R., and J. M. Buffington., 1998. Channel processes, classification, and
response in: R. J. Naiman and R. E. Bilby (eds.), River Ecology and Management: Lessons
from the Pacific Coastal Ecoregion. New York, Springer, pp13-42.
Németh, E., 1954. Hydrologyhidrometria. Budapest: Tankönyvkiadó. pp. 458 .
Peppler, M., 2006. Effects of Magnitude and duration of large floods on channel
Morphology: A case study of North fishcreek.Msc. Wisconsin – Madison University.
Railoun, M.Z., 2014. Plate [photographs] 1, 2, 3, 4, 5, 6. (Private collection – Honours
project), Somerset West.
Rosgen, D., 1996, Applied River Morphology, Colorado Pagosa Springs, Wildland
Hydrology, variable pagination.
Rowntree, K.M. and Wadeson, R.A., 1999. A Hierarchical Geomorphological Model for the
Classification of Selected South African Rivers. WRC Report No. 497/1/99. Water Research
Commission, Pretoria, South Africa.
Rowntree, K.M., 2000. Geography of drainage basins: hydrology, geomorphology, and
ecosystem management. In: Fox R and Rowntree KM (eds.). The Geography of South Africa
in a Changing World. Oxford University Cape Town Press,pp. 390-415.
Scheepers, A.C.T., 2014. Fluvial Geomorphology, 706 Fluvial Geomorphology. University
of the Western Cape, unpublished.
Schumm, S.A., and Lichty, R.W., 1965.Time, space, and causality in geomorphology, Amer.
J. Sci., 263, 110-119.
Schumm, S.A., 1977, The Fluvial System, New York: John Wiley and Sons, Inc., 338 p.
34. 34
Schick, A.P., 1988. Hydrologic aspects of floods in extreme arid environments, in: Baker VR,
Kochel RC, Patton PC (eds.), Flood Geomorphology. New York Wiley, pp. 189-203.
Simon, A., 1995. Adjustment and recovery of unstable alluvial channels:Identification and
approaches for engineering management.Earth Surf. Proc. Land. 20 611-628.
Soden, B.J.,2000. The sensitivity of the tropical hydrological cycle to ENSO. Journal of
Climate, 13, pp. 538–549.
Tharme R., Ractliffe, G., and Day, E., 1997.An Assessment of the Present Lourens River,
Western Cape, with Particular Reference to Proposals for Stormwater Management. Report to
the Somerset West Municipality. Southern Waters Ecological Research Consulting cc,
Freshwater Research Unit, University of Cape Town, Cape Town.
Thorne, C.R., 1982. Processes and mechanisms of river bank erosion. In: Hey RD, Barhurst
JC and Thorne CR (eds.) Gravel-Bed Rivers. USA, John Wiley and Sons Ltd,.pp. 227-259.
Thorne, C.R., 1990.The effect of vegetation on riverbank erosion and stability. In: Thornes
JB (ed.) Vegetation and Erosion. Chichester, John Wiley and Sons Ltd.,. pp. 125-144.
Thorne, C.R., Allen, R.G., and Simon, A., 1996. Geomorphological river channel
reconnaissance for river analysis, engineering and management. Transactions of the Institute
of British Geographers, New Series 21,pp 469-483.
Thorne, C.R., 1997. Channel Types and Morphological Classification. In Thorne, C. R., Hey,
R. D. and Newson, M. D. Applied Fluvial Geomorphology for River Engineering and
Management. Chichester John Wiley & Sons,pp 250-312.
Wolman, M. G. and J. P. Miller., 1960.Magnitude and Frequency of Forces in Geomorphic
Processes, Journal of Geology, 68, pp. 54-74.
35. 35
Appendix A
(a) Cross - section 1.1
(b) Cross – section 1.2
Figure 8(a-b): Cross-section survey at the study site during September 2014.
36. 36
(c) Cross – section 1.3
(d) Cross – section 1.4
(e) Cross – section 1.5
Figure 8(c-e): Cross-section survey at the study site during September 2014.
37. 37
(f) Cross - section 1.8
(g) Cross - section 1.9
Figure 8(f-g): Cross-section survey at the study site during September 2014.
38. 38
.
Plate 4 (a-h): Photographic monitoring of study site between cross section 1.3 and 1.4 (Photographs taken
between June – September2014); Plate 5(a-h): Photographic monitoring of study site at cross section 1.5. Note
the lateral migration of the left bank of plate 5a-f (Photographs taken between June – September 2014).
Plate 6 (a-f): Photographic monitoring of the study site at cross section 1.8 looking upstream, note the change
of hydraulic biotypes from June – September 2014 as well as vegetation which gets denser at each photo.
